We have derived a continuous line of human renal adenocarcinoma cells grown in vitro from a patient with renal cancer and erythrocytosis. The cells secrete into the growth medium and erythropoiesis stimulating factor which our results indicate is erythropoietin. We propose to clone this cell line and derive sublines with high erythropoietin secretory activity. These cells will permit us to isolate, purify to homogeneity and characterize substantial quantities of secretory erythropoietin and other secretory products and compare its (their) properties to those of human urinary erythropoietin by incubation of the cells with 3H, 14C, and/or 35S amino acids. These radioactive derivatives will serve as discrete probes of erythropoietin receptors on the red cell precursors of the marrow and will also make possible the development of a reliable and sensitive radio-immunoassay for the hormone.